BGDA Lecture - Development of the Embryo/Fetus 2
This lecture covers the period of Embryonic development, in Humans from week 3 to week 8 (GA week 5-10) and is divided into 23 Carnegie stages of embryonic development. There will also be a brief introduction to fetal development. Note, the period from week 9 to week 38 is considered Fetal development and will be covered in detail in the Laboratory 12.
- Understand key structures and events in embryonic development.
- Understanding of the dynamic changes internal and external structures.
- Brief understanding of organ and system formation (functional / not functional).
- Brief understanding of critical periods of development.
BGDA Practical Classes
|Practical 3 - Fertilization to Implantation||Practical 6 - Implantation to 8 Weeks||Practical 12 - Fetal Period|
First 8 Weeks
The Carnegie stages of the first 8 week of human development.
Mesoderm means the "middle layer" and it is from this layer that the body's connective tissues are derived (note that the head neural crest ectoderm also forms connective tissues)
In early mesoderm development a number of transient structures will form and then be lost as tissue structure is patterned and organised.
Humans as vertebrates have a "backbone" and the first mesoderm structure we will see form after the notochord will be somites.
Mesoderm organization: (left to right)
lateral plate - intermediate mesoderm - paraxial mesoderm - axial mesoderm - paraxial mesoderm - intermediate mesoderm - lateral plate
Adult - contributes to the nucleus pulposis of the intervertebral disc
Adult - metanephros forms the kidney
Lateral Plate Mesoderm
Somite initially forms 2 main components
- ventromedial- sclerotome forms vertebral body and intervertebral disc
- dorsolateral - dermomyotome forms dermis and skeletal muscle
- sclerotome later becomes subdivided
- rostral and caudal halves separated laterally by von Ebner's fissure
- half somites contribute to a single vertebral level body
- other half intervertebral disc
- therefore final vertebral segmentation “shifts”
- Body - epaxial and hypaxial muscles
- Limbs - flexor and extensor muscles
- connective tissue underlying epidermis
- begins as a dorsal thickening
- spreads throughout the body
|Heart Development Movies|
- forms initially in splanchnic mesoderm of prechordal plate region - cardiogenic region
- growth and folding of the embryo moves heart ventrallly and downward into anatomical position
- week 3 begins as paired heart tubes that fuse to form single heart tube
- begins to beat in Humans- day 22-23
- heart tube connects to blood vessels forming in splanchnic and extraembryonic mesoderm
Week 2-3 pair of thin -walled tubes
Week 3 tubes fused, truncus arteriosus outflow, heart contracting
Week 4 heart tube continues to elongate, curving to form S shape
Week 5 Septation starts, atrial and ventricular
- Links: Cardiac Embryology
- forms in the midline of the neural plate (day 18-19)
- either side of which are the neural folds which continues to deepen until about week 4
- neural folds begins to fuse, beginning at 4th somite level
Neural - 3 primary vesicles
- population of cells at the edge of the neural plate that lie dorsally when the neural tube fuses
- dorsal to the neural tube, as a pair of streaks
- pluripotential, forms many different types of cells
- cells migrate throughout the embryo
Neural Crest Derivatives: dorsal root ganglia, autonomic ganglia, adrenal medulla, drg sheath cells, glia, pia-arachnoid sheath, skin melanocytes, connective tissue of cardiac outflow, thyroid parafollicular cells, craniofacial skeleton, teeth odontoblasts
- branchial arch (Gk. branchia= gill)
- arch consists of all 3 trilaminar embryo layers (ectoderm- outside, mesoderm - core of mesenchyme, endoderm - inside)
- Humans have 5 arches - 1, 2, 3, 4, 6 (Arch 5 does not form or regresses rapidly)
- from in rostro-caudal sequence, Arch 1 to 6 from week 4 onwards
- arch 1 and 2 appear at time of closure of cranial neuropore
- Face - mainly arch 1 and 2
- Neck components - arch 3 and 4 (arch 4 and 6 fuse)
- During week 4 a series of thickened surface ectodermal patches form in pairs rostro-caudally in the head region.
- These sensory placodes will later contribute key components of each of our special senses (vision, hearing and smell).
- Note that their initial postion on the developing head is significantly different to their final position in the future sensory system
- Otic placode - istage 13/14 embryo the otic placode sunk from the surface ectoderm to form a hollow epithelial ball, the otocyst, which now lies beneath the surface surrounded by mesenchyme (mesoderm). The epithelia of this ball varies in thickness and has begun to distort, it will eventually form the inner ear membranous labyrinth.
- Lens placode - lies on the surface, adjacent to the outpocketing of the nervous system (which will for the retina) and will form the lens.
- Nasal placode - has 2 components (medial and lateral) and will form the nose olefactory epithelium.
Upper and Lower Limb
- Begins at buccopharyngeal membrane
- Ends at cloacal membrane
- 3 distinct portions (fore-, mid- and hind-gut)
- liver earliest forming organ
Germ layer contributions
- Endoderm - epithelium and associated glands
- Mesoderm (splanchnic) - mesentry, connective tissues, smooth muscle, blood vessels
- Ectoderm (neural crest) - enteric nervous system
Both endoderm and mesoderm will contribute to associated organs.
- Heart - septation starts, atrial and ventricular
- Vascular - 3 vascular systems (systemic, placental, vitelline) extensively remodelled
- Respiratory - left and right lung buds push into the pericardioperitoneal canals (primordia of pleural cavity)
- Sense - Hearing cochlear part of otic vesicle elongates (humans 2.5 turns)
Septation continues, atrial septa remains open, foramen ovale
- Endocrine development
- Pituitary - connecting stalk between pouch and oral cavity degenerates
- Parathyroid - diverticulum elongate, hollow then solid, dorsal cell proliferation
- Thymus - diverticulum elongate, hollow then solid, ventral cell proliferation
- Adrenal - fetal cortex forms from mesothelium adjacent to dorsal mesentery, medulla neural crest cells from adjacent sympathetic ganglia
- Pancreas - Week 7 to 20 pancreatic hormones secretion increases, small amount maternal insulin
- Limb bones form by endochondrial ossification and throughout embryo replacement of cartilage with bone (week 5 onward).
- Limb upper and lower limbs rotate in different directions (upper limb dorsally, lower limb ventrally)
- First Trimester (1 - 12 weeks) - embryonic and early fetal
- Second Trimester (13 - 24 weeks) - organ development and function, growth (length)
- Third Trimester (25 - 40 weeks) - organ function and rapid growth (weight)
- During the fetal period there is ongoing growth in size, weight and surface area of the brain and spinal cord. Microscopically there is ongoing: cell migration, extension of processes, cell death and glial cell development.
- Brain - folding of the initially smooth surface (Insular cortex, Gyral and Sulcal development)
- Neural development will continue after birth with substantial growth, death and reorganization occuring during the postnatal period
- Links: Neural System Development
- week 4 - 5 embryonic
- week 5 - 17 pseudoglandular
- week 16 - 25 canalicular
- week 24 - 40 terminal sac
- late fetal - 8 years alveolar
- ovary and testis development
- external genital development
- testis descent
- Links: Genital System Development
- week 32-34 nephron development completed
- term birth nephron number per kidney about 1 million (300,000 to 2 million)
- Links: Renal System Development
- Many endocrine organs begin to function in the early fetal period.
- Pituitary Hormones - HPA axis established by week 20, pituitary functional throughout fetal development
- Thyroid Hormone - important for neural development, required for metabolic activity, also in the newborn
Remember that the Placenta also has important endocrine functions during development.
The term "Critical Periods" refers to periods of development when specific systems are more sensitive to teratogen exposure or developmental insults.
|Conceptus||Embryonic development (weeks)||Fetal period (weeks)|
|Loss||Major abnormalities||Functional and Minor abnormalities|
- To see more details about 2nd and 3rd trimester development see Fetal Development.
- To understand dynamic changes in structures see Movies.
- The associated BGDA Practical 6 class.
Carnegie Stage Table
Weeks shown in the table below are embryonic post ovulation age, for clinical Gestational Age (GA) measured from last menstrual period, add 2 weeks.
(not to scale)
|fertilized oocyte, zygote, pronuclei|
|morula cell division with reduction in cytoplasmic volume, blastocyst formation of inner and outer cell mass|
|loss of zona pellucida, free blastocyst|
|extraembryonic mesoderm, primitive streak, gastrulation|
|gastrulation, notochordal process|
|primitive pit, notochordal canal|
|Somitogenesis Somite Number 1 - 3 neural folds, cardiac primordium, head fold|
|Somite Number 4 - 12 neural fold fuses|
|Somite Number 13 - 20 rostral neuropore closes|
|Somite Number 21 - 29 caudal neuropore closes|
|Somite Number 30 leg buds, lens placode, pharyngeal arches|
|lens pit, optic cup|
|lens vesicle, nasal pit, hand plate|
|nasal pits moved ventrally, auricular hillocks, foot plate|
|straightening of trunk|
|upper limbs longer and bent at elbow|
|hands and feet turned inward|
|eyelids, external ears|
|rounded head, body and limbs|
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Cite this page: Hill, M.A. (2015) Embryology BGDA Lecture - Development of the Embryo/Fetus 2. Retrieved November 25, 2015, from https://embryology.med.unsw.edu.au/embryology/index.php/BGDA_Lecture_-_Development_of_the_Embryo/Fetus_2
- © Dr Mark Hill 2015, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G